Can't see your photo for some reason. \\; Could be either the black plastic (rounded/contoured) sleeve that goes between the throttle lock screw and the throttle sleeve. \\; It's black, rounded/countoured, about an in. long, maybe a 1/4 in. wide????
 \\;
Could also be the v shaped metal (sprung) spacer that serves to hold the brake light censor in position. \\;
 \\;
Maybe someone else will have luck w/ your photo................
it sits inside the right side \\;housing below grip \\;on the throttle lock screw end. it cradles the throttle from the bottom. when you screw the lock in, that piece raises and clamps the throttle so it wont move.
Harley calls it a "shoe" in the manual. Usually stays in place if you don't invert the lower half of the housing. Make sure you wedge the brake lever until it's back on and tightened or you'll be installing a brakelight switch.